Sidestep Slow Hosts

A slow host is one that requires more than a few seconds to accept delivery of a modestly sized email message. To illustrate, consider the following example produced by a verbose transaction of sending email to such a slow host:

% /usr/sbin/sendmail -v -q
Running /var/spool/mqueues/q.2/df/f0DHnvO02567 (sequence 1 of 1) Connecting to via esmtp...
220 ESMTP Sendmail 8.10.1/8.10.1; Fri, 13 Dec 2002 10:50:20 -0700 
>>> EHLO Hello [123.45.678.9], pleased to meet you
250 HELP
>>> MAIL From:<> SIZE=16
You wait 2 minutes for slowhost to look you up.

This situation can get worse, especially if the slow site runs slow antispam software, because that site can take 9 or 10 minutes to validate you. This can cause sendmail to seem to hang for 9 or 10 minutes, then suddenly to continue with:

250 2.1.0 <>... Sender ok
>>> RCPT To:<>
250 2.1.5 <>... Recipient ok
>>> DATA
354 Enter mail, end with "." on a line by itself
>>> .

Furthermore, some mail transfer agents (MTAs) start to place a message on disk only after all the data has been received, so writing to an NFS-mounted disk can appear to hang for several seconds:

250 2.0.0 f0DHoNh91321 Message accepted for delivery Sent (f0DHoNh91321 ...

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